Traditionally, exciters for synchronous machines are mounted near the stator of the synchronous machine with electrical connection to the field winding, usually found on the rotor. The function of the exciter is to provide electrical current to the field winding and thereby produce a magnetic field required for machine operation. These field exciters may be thought of as direct current (DC) controlled amplifiers of relatively high current rating and bandwidth with the role to control the machine output voltage. These amplifiers may use, among other power electronics, thyristor bridges fed directly from a three-phase transformer connected to a grid or directly from the machine terminals to generate the DC voltage feeding the machine's exciter winding. When used to supply machines used for power generation, the field exciters are generally large in size and weight due in part to the weight of their magnetic components. For example, airplanes that have large power requirements are directly impacted by the size, output power and weight of electrical generators and their accompanying field exciters. If airplanes continue to have larger power requirements, the size and weight of power generating synchronous machines will be an increasing issue.
Attempts in the past to resolve the issue of size and weight of synchronous machines have succeeded in some respects and failed in other respects. A relatively recent idea is cryogenically cooling part of the synchronous machine including the field exciter and portions may be made superconducting. It is generally accepted that cryogenic cooling is around 100 K (Kelvin) and below. At low temperatures, the current handling capabilities and switching speeds of many semiconductor devices improve and the resistance of coils and conductors are reduced, with superconductors obtain zero resistance at DC; thus, size and weight of these components is reduced. Although this approach reduces the size and weight of components and improves performance, it may prove uneconomical because cooling may do little to reduce some components' size and weight or the cooling is too expensive.